1. Field of the Invention
This invention relates to a method of and apparatus for generating fluorescence diagnostic information reflecting properties of tissue of an object part on the basis of fluorescence emitted from the object part.
2. Description of the Related Art
There has been proposed a fluorescence diagnostic information generating apparatus which projects stimulating light of a predetermined wavelength onto an object part such as an organic body and outputs fluorescence diagnostic information such as a fluorescence diagnostic image representing properties of tissue of the object part on the basis of fluorescence emitted from the object part. Such fluorescence diagnostic information generating apparatuses can be divided into those which output fluorescence diagnostic information on the basis of fluorescence emitted from the object part which has been caused to absorb a fluorescence agent and those which output fluorescence diagnostic information on the basis of autofluorescence emitted from the object part itself without use of a fluorescence agent. In this specification, the fluorescence emitted from the object part which has been caused to absorb a fluorescence agent will be sometimes referred to as “the agent fluorescence” and the fluorescence diagnostic information generating apparatus of the former type will be sometimes referred to as “the agent fluorescence diagnostic information generating apparatus”, hereinbelow. Whereas, the fluorescence diagnostic information generating apparatus of the latter type will be sometimes referred to as “the autofluorescence diagnostic information generating apparatus”, hereinbelow. The fluorescence diagnostic information generating apparatus is generally incorporated in an endoscope which is inserted into a body cavity, a colposcope, a surgical microscope or the like.
For example, the fluorescence diagnostic information generating apparatus includes an apparatus which outputs a fluorescence diagnostic image on the basis of the intensity of fluorescence emitted from an organic tissue which has been exposed to stimulating light. As shown in FIG. 13, the intensity of fluorescence emitted from a diseased tissue is lower than that of fluorescence emitted from a normal tissue. Accordingly, normal tissues can be distinguished from diseased tissues on the basis of fluorescence diagnostic image based on the intensity of fluorescence.
In the case where the fluorescence diagnostic information generating apparatus is incorporated in an endoscope, the distance between the stimulating light source and the object part cannot be uniform due to the protruding portions and the recessed portions of the object part and the illuminance of the stimulating light at the surface of the object part cannot be uniform. Generally intensity of fluorescence emitted from a normal tissue is proportional to the illuminance of the stimulating light, and the illuminance of the stimulating light reduces in inverse proportion to the square of the distance. Accordingly, fluorescence emitted from a diseased tissue at a smaller distance from the stimulating light source can be higher in intensity than that emitted from a normal tissue at a larger distance from the stimulating light source, and judgment on the properties of the object tissue solely on the basis of the intensity of fluorescence can result in an erroneous judgment.
In order to avoid such an erroneous judgment, there has been proposed a fluorescence diagnostic information generating apparatus which outputs the fluorescence diagnostic information on the basis of the yield of fluorescence which is a value based on the ratio of the intensity of the stimulating light which the object part receives to the intensity of fluorescence emitted from the object part and does not depend upon the distance or the angle at which the stimulating light is projected onto the object part.
However, since the stimulating light in an ultraviolet region to a visible region is absorbed in different amounts by different substances of the object part, the intensity distribution of the stimulating light projected onto the object part cannot be accurately measured by measuring the intensity distribution of the reflected stimulating light, and accordingly, it has been difficult to obtain the yield of fluorescence on the basis of the intensity distribution of the reflected stimulating light.
As a method of obtaining the yield of fluorescence (or a value reflecting the yield of fluorescence), there has been proposed a method in which near-infrared light which undergoes more uniform absorption as compared with the stimulating light in an ultraviolet region to a visible region is projected onto the object part as reference light, and the value obtained by dividing the intensity of fluorescence by the intensity of the reflected near-infrared light is employed as a value reflecting the yield of fluorescence. That is, by obtaining the value reflecting the yield of fluorescence, terms representing the intensity of fluorescence depending upon the distance between the stimulating light source and the object part and/or the distance between the fluorescence receiving portion and the object part are cancelled.
Further, the shape of spectrum of fluorescence emitted from a normal tissue differs from that of fluorescence emitted from a diseased tissue as shown in FIG. 13. A fluorescence diagnostic information generating apparatus based on the difference has been developed. For example, as disclosed in Japanese Unexamined Patent Publication No. 6(1994)-54792, there is proposed an apparatus which outputs fluorescence diagnostic information on the basis of a G/R ratio, the ratio of the intensity of fluorescence in a green wavelength band to the intensity of fluorescence in a red wavelength band.
Further, we, this applicant, have proposed, in Japanese Unexamined Patent Publication No. 10(1998)-225436, an apparatus which outputs fluorescence diagnostic information reflecting a normalized intensity of fluorescence, an intensity of narrow wavelength band light from the object part normalized by an intensity of broad wavelength band light from the object part. In the apparatus, image data in a narrow wavelength band near 480 nm and image data in a broad wavelength band from near 430 nm to near 730 nm are obtained and a normalized intensity of fluorescence is obtained by dividing the values of pixels in the image data in the narrow wavelength band by the values of pixels in the image data in the broad wavelength band. Then a pseudo-color image based on the normalized intensity of fluorescence, for instance, a color image in which color changes from green to red as the normalized intensity of fluorescence is reduced, is output as a fluorescence diagnostic image. That is, by obtaining the normalized intensity of fluorescence, terms representing the intensity of fluorescence depending upon the distance between the stimulating light source and the object part and/or the distance between the fluorescence receiving portion and the object part are cancelled, a fluorescence diagnostic image based on solely values reflecting the difference in fluorescence spectrum can be output.
The object tissue is sometimes stained with disturbance factors such as blood, mucus, digestive fluid, saliva, foam, residue and the like. When an organic tissue stained with such a disturbance factor (will be referred to as “unclean tissue”, hereinbelow) is exposed to the stimulating light, the disturbance factor also emits fluorescence. Fluorescence emitted from an unclean tissue is sometimes confusing with fluorescence emitted from a diseased tissue in the normalized intensity of fluorescence and/or the yield of fluorescence.
Generation of fluorescence diagnostic image on the basis of the normalized intensity of fluorescence will be described, hereinbelow, by way of example. FIG. 14 is a view showing the shapes of spectra of fluorescences emitted from a clean normal tissue (the dotted line), a clean diseased tissue (the chained line) and an unclean tissue (the solid line) upon exposure to stimulating light of 410 nm, wherein the intensity of fluorescence is normalized so that the integrated value becomes 1.
In the case where image data in a narrow wavelength band near 480 nm and image data in a broad wavelength band from near 430 nm to near 730 nm are obtained, a normalized intensity of fluorescence is obtained by dividing the values of pixels in the image data in the narrow wavelength band by the values of pixels in the image data in the broad wavelength band, and a pseudo-color image based on the normalized intensity of fluorescence, for instance, a color image in which color changes from green to red as the normalized intensity of fluorescence is reduced, is output as a fluorescence diagnostic image, pseudo-color is generally allocated so that the normalized intensity of fluorescence emitted from the normal tissue is displayed in green and the normalized intensity of fluorescence emitted from the diseased tissue is displayed in red. Generally, normalized intensities of fluorescence not higher than the normalized intensity of fluorescence emitted from the diseased tissue are all displayed in red. Since the normalized intensity of fluorescence emitted from the unclean tissue is generally close to or lower than the normalized intensity of fluorescence emitted from the diseased tissue, the normalized intensity of fluorescence emitted from the unclean tissue is also displayed in red. That is, both the normalized intensity of fluorescence emitted from the diseased tissue and the normalized intensity of fluorescence emitted from the unclean tissue are displayed in red. When the viewer distinguishes normal tissues from diseased tissues on the basis of a fluorescence diagnostic image thus obtained, the viewer can mistake an unclean tissue for a diseased tissue, whereby the tissue-property distinguishing accuracy is deteriorated.
Since many of unclean tissues emit weak fluorescence as a clean diseased tissue, even if the fluorescence diagnostic image is generated on the basis of the intensity of fluorescence or the yield of fluorescence, the viewer can mistake an unclean tissue for a diseased tissue, whereby the tissue-property distinguishing accuracy is deteriorated.